Superconductive alloys



United States Patent M 3,294,529 SUPERCONDUCTIVE ALLOYS Robert C.Haverstraw, Willoughby, Ohio, and Malcolm J.

Fraser, Penn Hills, Pittsburgh, Pa., assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of Pennsylvania NoDrawing. Filed Sept. 30, 1963, Ser. No. 312,309 5 Claims. (Cl. 75-134)This invention relates to the preparation of superconductive alloys andin particular comprises a method to improve cryogenic properties ofsuperconductive alloys as well as to provide novel superconductivealloys.

A major goal in research in superconductive alloys is the improvement ofthe critical current density at high magnetic fields. Changes in alloycontent as Well as the use of different alloying constituents are waysin which improved critical current densities have been obtained.However, upon making such changes, production problems are changed andmay create insurmountable obstacles to the use of an otherwise promisingmaterial. Accordingly, it is highly desirable to find ways of improvingcryogenic properties in superconductive alloys that are known to befabricable or to provide improved cryogenic properties in ways such thatfabrication problems are not encountered.

It is therefore a primary object of the present invention to provide amethod whereby the cryogenic properties of superconductive alloys can beimproved by an easily practiced procedure that does not deleteriouslyaffect the mechanical properties of the materials.

A further object of the invention is to provide a method in accordancewith the foregoing object in which fabrication problems are notcomplicated and there result superconductive materials having improvedcryogenic properties.

Another object of the invention is to provide new superconductivematerials.

Other objects and advantages of the invention will appear from thefollowing detaileddescription and discusmen.

It has now been discovered, and it is on this discovery that theinvention is in large part predicated, that upon introducing hydrogeninto superconductive materials, as described more particularlyhereinafter, there results a superconductive material comprising themetal constituents and hydrogen having an increased volume. Upondetermining, for example, the critical current density of thesuperconductive material before and after the hydrogen addition, it isfound that a surprising increase therein results. Moreover, since theintroduction of hydrogen can be practiced on the superconductivematerial after it has been worked to the desired state, it is evidentthat loss, if any, in mechanical properties need not prevent use of theimproved superconductive material.

The invention can be practiced with superconductive alloys and compoundsgenerally. This includes alloys and compounds of superconducting,exothermic, hydrogen occluding elements. These elements are in twogroups. The first group includes titanium, vanadium, niobium, zirconium,lanthanum, tantalum, thorium and uranium, while the second groupincludes aluminum, zinc, gallium, cadmium, indium, tin, mercury,thallium and lead. The compounds and alloys can be composed of 1) anytwo or more elements of the first group, or (2) one or more elements ofthe first group with any one or more elements of the second group. Thepreferred superconductive materials with which the invention can bepracticed are the zirconium-niobium alloys in which zirconium may rangefrom about to 65, and preferably to 50, weight percent, and thebimetallic compound of niobium and tin,

3,294,529 Patented Dec. 27, 1966 Nb Sn, since the former of these can befabricated to fine Wire or other shape readily and the latter has thehighest known critical current density attained prior to the presentinvention. These materials may be used in the invention in theconventional forms now used in superconducing applications, with thinwires on the order of 0.015 inch and smaller down to about 0.001 inch indiameter being preferred. The shapes can be formed in the usual mannerof alloying the metals, preferably in a purity of at least 99 weightpercent, and then working the resulting material to final size and shapeby practices presently known, for example, hot and cold working at lightpasses, the latter suitably using intermediate annealing steps.

In the invention hydrogen is added to superconductive material byelectrolytically liberating it on the superconductor while the latter isa cathode in an electrolytic system. Accordingly, an electrolyte thatliberates hydrogen upon electrolysis is required. Any hydrogenliberating electrolyte is operative in the invention, but it ispreferred to use aqueous mineral acid solutions of, for example,hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid andmixtures thereof. Preferably, an aqueous electrolyte having an acidconcentration within the range of about 5 to 25 weight percent is used.The usual parameters in electrolysis, such as the electrolyteconstitution and concentration, current density and the like, vary theeffectiveness of the hydrogen charging process and are not independentlycritical. Generally a current density on the order of 0.1 to 5 amperesper square centimeter of cathode is used. As will be apparent to theartisan, the best conditions of operation as well as electrolyteconstitution will be largely dictated by the specific application andmaterials involved in any given process. Platinum and stainless steelare the preferred materials for the anode in these cells, but othermaterials can be used as well.

The amount of cathodic charging that can be practiced in thi sinventionis limited by embrittlement of the superconductive cathode from hydrogenocclusion. That, of course, is a practical limitation rather than alimitation affecting operability. Generally cathodic charging isconducted sufficiently to cause a volume increase in the superconductorbeing treated of about 5 to 15 percent or more, based on the originalvolume. It is to be noted that the volume change adds considerably tothe plastic deformation of the matrix. While reference 'has been madeabove to the hydrogen addition as being an occlusion phenomenon, itshould be understood that this has been done for purposes of discussionand our invention is not to be limited by any theory regarding the,nature of the hydrogen retention. Whatever the manner by which thehydrogen is held, there result new superconductive materials composed ofhydrogen and the two metals. These materials are stable at ordinaryconditions and it is expected that stability will be retained for anumber of years.

The invention will be described further in conjunction with thefollowing examples in which the details are given by way of illustrationand not by way of limitation.

An 0.010 inch diameter cold drawn Wire having a composition, by weight,of 25 percent zirconium and the remainder niobium was used. This Wirewas connected as a cathode to a DC. power source and placed in anelectrolyte formed from a concentrated acid mixture of, by weight, 1part of hydrofluoric acid, 3 parts of nitric acid and 5 parts ofsulfuric acid, the acid mixture having been diluted with Water to a 10weight percent solution. A platinum electrode was :used as the anode.The cathode was then hydrogen charged in accordance with this inventionby passing current through the cell at a current density, measured atthe cathode, of 1 ampereper square centimeter.

The foregoing procedure was carried out on several samples. A firstsample was charged for a total of 3 minutes under the above conditions.In another run a sample was charged for a period of 6 minutes. Inaddition, another sample was changed for two 3-minute cycles. Thecritical current density (in amperes) at fields of 5 to 20 kilogauss wasdetermined on each sample before and after each charging cycle. The dataobtained are:

It should be noted that these tests were of a qualitative rather than ofa quantitative nature and no attempts were made to produce the highestpossible current density. The above data demonstrate markedly thatcathodic charging with hydrogen produce a significant increase in thecritical current density at each of the fields tested. At the 20kilogauss field and with 6 minutes of the cathodic charging, an increasein the critical current density of as high as 30 percent occurred. It isthus apparent that the invention can produce significant improvements inthe critical current density of these materials.

To determine whether or not hydrogen charging destroyed the mechanicalproperties of the material, ductility tests were conducted on the samplecharged for the 6-minute cycle. It was found that it could be bent to aradius at least as small as 0.005 inch without breakage. This sample wasthen cold reduced by rolling it to a ribbon 0.0025 inch thick and 0.025inch wide. The critical current (in amperes) was again determined atfields of 5 to 20 kilogauss. In addition, a sample of the same 0.010inch diameter wire that had not been charged was similarly cold reducedand its critical current (amperes) determined. The data obtained are:

Though the cold rolling was conducted to see if the charged materialcould be further worked, we were surprised to find further markedimprovements in the critical current as shown by the above data. Thevalues transverse to the rolled surface should be compared with thoseunder sample 2 in Table I. Thus the invention provides an additional Wayby which superconductive materials with improved critical currentdensity can be obtained.

The hydrogen charging of the other superconductive materials is carriedout in the same general fashion but with suitable variations ofelectrolyte composition to avoid unnecessary attack on thesuperconductive cathode. It is to be noted that even Nb Sn, the materialwith the highest known critical current density, can be improved inaccordance with our discoveries, that is by cathodically charging itwith hydrogen in aqueous mineral acid electrolyte.

From the foregoing discussion and description, it is manifest that amarkedly simple and effective procedure is provided by which thecritical current density of a superconductive material can be improved.

While the invention has been described with regard to detailed specificembodiments, it will be understood that variations, substitutions,changes and the like can be made without departing from its scope.

We claim:

1. A method of improving the cryogenic properties of superconductivematerials comprising electrolytically liberating hydrogen on saidsuperconductive material while said material is disposed as a cathode ina hydrogen liberating electrolyte, the said superconductive materialbeing composed of at least one member selected from the group consistingof titanium, vanadium, niobium, zirconium, lanthanum, tantalum, thoriumand uranium and one diflerent member of the group consisting of each ofthe members of the foregoing group and aluminum, zinc, gallium, cadmium,indium, tin, mercury, thallium and lead, and recovering the resultinghydrogen occluding superconductive material.

2. A method of improving the cryogenic properties of superconductivematerials comprising electrolytically liberating hydrogen on saidsuperconductive material while said material is disposed as a cathode ina hydrogen liberating electrolyte, said superconductive materialcomprising an alloy of 10 to weight percent zirconium and the remainderniobium, and recovering the resulting hydrogen occluding superconductivealloy.

3. A method in accordance with claim 2 in which the zirconium-niobiumalloy superconductor is in wire form and is hydrogen charged until itsvolume increases up to about 15 percent.

4. A process in accordance with claim 3 in which the chargedzirconium-niobium alloy superconductor is then cold worked to furtherincrease the cryogenic properties.

5. A new superconductor comprising hydrogen and a zirconium-niobiumalloy in which the zirconium is present to an amount of 15 to 60 weightpercent of the alloy, the hydrogen being present in an amount sutficientto provide a volume in the superconductor of about 5 to 15 percentgreater than that of the Zirconium-niobium alloy free from the hydrogen.

References Cited by the Examiner UNITED STATES PATENTS 1/1957 Brown 204-OTHER REFERENCES DAVID L. RECK, Primary Examiner.

W. C. TOWNSEND, Examiner. v C. N. LQVELL, Assistant Examiner.

5. A NEW SUPERCONDUCTOR COMPRISING HYDROGEN AND A ZIRCONIUM-NIOBIUM ALLOY IN WHICH THE ZIRCONIUM IS PRESENT TO AN AMOUNT OF 15 TO 60 WEIGHT PERCENT OF THE ALLOY, THE HYDROGEN BEING PRESENT IN AN AMOUNT SUFFICIENT TO PROVIDE A VOLUME IN THE SUPERCONDUCTOR OF ABOUT 5 TO 15 PERCENT GREATER THAN THAT OF THE ZIRCONIUM-NIOBIUM ALLOY FREE FROM THE HYDROGEN. 